Cyclic hexapeptides having endothelin antagonistic activity

Abstract: A cyclic hexapeptide represented by the formula: cyclo[A-Asp(R¹)-Y-NH-CHR²-CO-C-D-Trp(N in -R³)-] (I) wherein A is a D-acidic-α-amino acid residue; Y is an acidic-α-amino acid residue; C is an L-α-amino acid residue; R¹ is a group represented by the formula: wherein X¹ and X² are independently H, C₁₋₆ alkyl, C₁₋₆ alkoxy, halogen or nitro, and may bind together to form a ring in cooperation with the adjacent carbon atom; R² is C₁₋₆ alkyl, C₃₋₇ cycloalkyl, C₃₋₇ cycloalkyl-C₁₋₃ alkyl, C₁₋₆ alkylthio-C₁₋₃ alkyl, C₃₋₇ cycloalkylthio-C₁₋₃ alkyl, C₁₋₅ alkoxy-C₁₋₃ alkyl, C₃₋₇ cycloalkoxy-C₁₋₃ alkyl, C₁₋₆ alkylthio, C₃₋₇ cycloalkylthio, C₁₋₅ alkoxy or C₃₋₇ cycloalkoxy; R³ is H, C₁₋₆ alkyl, C₃₋₇ cycloalkyl, -COR⁴, - COOR⁵ or -CONHR⁶; when X¹, X² and R³ are all hydrogen atoms, R² is not an isobutyl group, or a saalt thereof, which exhibits excellent antagonistic action against endothelin receptors, particularly endothelin B receptors.

Description: BACKGROUND OF THE INVENTION The present invention provides a cyclic hexapeptide or a salt thereof, a method of its production, and an endothelin receptor antagonist containing said cyclic hexapeptide or a pharmacologically acceptable salt thereof. Endothelin (ET), a vasoconstrictive peptide comprising 21 amino acids, is isolated from the culture supernatant of the endothelial cells of porcine aortas and structurally determined by Yanagisawa et al. in 1988 [Yanagisawa et al., Nature, Vol. 332, pp. 411-412]. Research into endothelin-encoding genes has demonstrated that endothelin occurs in 3 structurally similar peptides, designated endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3), respectively. On the other hand, two types of endothelin receptors are known: endothelin A receptors, which show higher affinity for ET-1 and ET-2 than for ET-3, and endothelin B receptors, which show equivalent affinity for ET-1, ET-2 and ET-3. In the part, endothelin A receptors were thought to be present in vascular smooth muscle and to induce smooth muscle contractions, endothelin B receptors to be present in vascular endothelial cells and to induce smooth muscle relaxation. In recent years, however, endothelin B receptors have been shown to be involved in vascular smooth muscle contractions as well [Biochemical and Biophysical Research Communications, Vol. 175(2), 556-561 (1991); Journal of Cardiovascular Pharmacology, Vol. 20 (Suppl. 12), S11-S14 (1992)]. There is therefore a need for the development of endothelin B receptor antagonists, as well as endothelin A receptor antagonists. Since the development of endothelin, there have been intensive investigations in search of compounds that antagonize endothelin receptors, with the aim of developing therapeutic agents for diseases caused by endothelin. For example, cyclic peptides showing endothelin receptor antagonistic action are disclosed in EP0436189A1 and EP0528312A2. The cyclic peptide of Japanese Patent Unexamined Publication No. 261198/1992, in particular, is known to be very low in affinity for endothelin B receptors, though it has affinity for endothelin A receptors. The compound disclosed in Japanese Patent Unexamined Publication No. 261198/1992 is therefore faulty in that vascular smooth muscle contractions mediated by endothelin B receptors are not suppressed satisfactorily, though it suppresses vascular smooth muscle contractions mediated by endothelin A receptors. On the other hand, the compound disclosed in EP0528312A2 is known to be an excellent endothelin receptor antagonist in that it has affinity not only for endothelin A receptors but also for endothelin B receptors. However, there is a need for development of an endothelin B receptor antagonist that shows better affinity for endothelin B receptors, so as to efficiently suppress vascular smooth muscle contractions mediated by such receptors. The object of the present invention is to provide a compound having the antagonistic activity on endothelin B receptors with greater potency than for other types of endothelin receptors. SUMMARY OF THE INVENTION To solve the above problems, the present inventors investigated in search of a compound having the antagonistic activity on endothelin B receptors with greater potency than for other types of endothelin receptors, and found that particular cyclic hexapeptides included in the scope of claims of EP-A-0 528 312 but not specifically disclosed in the Examples given in the specification therefore, exhibit more potent endothelin B receptor antagonistic action than the cyclic hexapeptides or salts thereof specifically disclosed said in Examples. The inventors made further investigations based on this finding, and developed the present invention. Accordingly, the present invention relates to (1) a cyclic hexapeptide represented by the formula (I): cylco[-D-Asp-Asp(R 1 )-L-Asp-NH-CHR 2 -CO-L-Leu-D-Trp(N in -R 3 )-] wherein R 1 is a group represented by the formula: wherein X 1 is a hydrogen atom, a C 1-6 alkyl group, a C 1-6 alkoxy group, a halogen atom or a nitro group; R 2 is a neopentyl group; and R 3 is a hydrogen atom or -COR 4 (R 4 represents a hydrogen atom, a C 1-6 alkyl group, a C 6-15 aryl group or a C 6-15 aryl-C 1-3 alkyl group); or a salt thereof; (2) a method of producing the cyclic hexapeptide defined in (1) above, which method comprises condensing two kinds of fragments of a cyclic hexapeptide represented by the formula: cylco[-D-Asp-Asp(R 1 )-L-Asp-NH-CHR 2 -CO-L-Leu-D-Trp(N in -R 3 )-] wherein R 1 , R 2 and R 3 have the same meaning as defined in (1) above, divided at an optically chosen position to form a linear hexapeptide, and then cyclizing the resulting linear hexapeptide to form the cyclic hexapeptide; and (3) the use of the cyclic hexapeptide as defined in (1) above or a pharmacologically acceptable salt thereof for the manufacture of a medicament for treating a disease caused by endothelin. DESCRIPTION OF THE PREFERRED EMBODIMENTS With respect to formula (I) above, the C 1-6 alkyl group for X 1 is exemplified by methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl and n-hexyl. C 1-3 alkyl groups, such as methyl, ethyl, n-propyl and isopropyl, are preferred, with greater preference given to methyl. The C 1-6 alkoxy group for X 1 is exemplified by methoxy, ethoxy, n-propoxy, n-butoxy, n-pentyloxy and n-hexyloxy. C 1-3 alkoxy groups, such as methoxy, ethoxy and n-propoxy, are preferred, with greater preference given to methoxy and ethoxy. The halogen atom for X 1 is exemplified by fluoro, chloro, bromo and iodo, with preference given to chloro. Specifically, R 1 above is exemplified by the following: The above group -Asp(R 1 )- includes the D-, L- and DL-configurations, with preference given to the L-configuration. With respect to formula (I) above, R 2 in -NH-CHR 2 -CO - represents a neopentyl group. The α-amino acid residue represented by -NH-CHR 2 -CO- above includes the D-, L- and DL-configurations, with preference given to the D-configuration. With respect to formula (I) above, R 3 in -D-Trp(N in -R 3 )- represents a hydrogen atom or -COR 4 (R 4 represents a hydrogen atom, a C 1-6 alkyl group, a C 6-15 aryl group or a C 6-15 aryl-C 1-3 alkyl group). The C 1-6 alkyl group for R 4 is exemplified by methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, (1-methyl)propyl, tert-butyl, n-pentyl, (2-methyl)butyl, (3-methyl)butyl, neopentyl, n-hexyl, (2,2-dimethyl)butyl and (3,3-dimethyl)butyl, with preference given to C 1-3 alkyl groups, such as methyl, ethyl, n-propyl and isopropyl. The C 6-15 aryl group for R 4 is exemplified by phenyl, α-naphthyl and β-naphthyl, with preference given to phenyl. The C 6-15 aryl-C 1-3 alkyl group for R 4 is exemplified by benzyl, phenylethyl, phenylpropyl, α-naphthylmethyl, α-naphthylethyl, α-naphthylpropyl, β-naphthylmethyl, β-naphthylethyl and β-naphthylpropyl, with preference given to C 6-15 arylmethyl groups, such as benzyl, α-naphthylmethyl and β-naphthylmethyl. Specifically, -COR 4 is exemplified by formyl, acetyl, propionyl, butyryl, isobutyryl, isovaleryl, pivaloyl, n-pentylcarbonyl, benzoyl and phenylacetyl. Preferable groups for R 3 include the hydrogen atom, formyl group and acetyl group. Moreover, it is preferable that X 1 is a hydrogen atom or a C 1-6 alkoxy group and R 3 is a hydrogen atom or -COR 4 (R 4 is a hydrogen atom or a C 1-3 alkyl group). The cyclic hexapeptide having a hydrogen atom for X 1 in R 1 and for R 3 and an isobutyl group for R 2 is produced in Example 70 of EP-A-0 528 312. The cyclic hexapeptide represented by the formula (I) or a salt thereof of the present invention, is exemplified by: cyclo[-D-Asp-L-Asp(B1)-L-Asp-D-γMeLeu-L-Leu-D-Trp-], cyclo[-D-Asp-L-Asp(B7)-L-Asp-D-γMeLeu-L-Leu-D-Trp-] and cyclo[-D-Asp-L-Asp(B7)-L-Asp-D-yMeLeu-L-Leu-D-Trp(Ac)-]. (See for B 1 and B 7 the list of abbreviations). Salts (or pharmacologically acceptable salts) of the cyclic hexapeptide (I) of the present invention include metal salts (e.g., sodium salt, potassium salt, calcium salt, magnesium salt, zinc salt), salts with bases or basic compounds (e.g., ammonium salt, arginine salt), inorganic acid adduct salts (e.g., hydrochloride, sulfate, phosphate) and organic acid salts (e.g., acetate, propionate, citrate, tartrate, malate, oxalate). Abbreviations for amino acids, peptides and others used in the present specification are based on abbreviations specified by the IUPAC-IUB Commission on Biochemical Nomenclature or abbreviations in common use in relevant fields. Some examples are given below. Gly : Glycine Sar : Sarcosine (N-methylglycine) Ala : Alanine Val : Valine Nva : Norvaline Ile : Isoleucine aIle : Alloisoleucine Nle : Norleucine Leu : Leucine N-MeLeu : N-methylleucine tLeu : Tertiary leucine γMeLeu : Gamma methylleucine Met : Methionine Arg : Arginine Lys : Lysine Orn : Ornithine His : Histidine Asp : Aspartic acid Asp (B1) : Aspartic acid β-4-phenylpiperazine amide Asp (B7) : Aspartic acid β-4-(2-methoxyphenyl)piperazine amide Glu : Glutamic acid Cys : Cysteine Cta : Cysteic acid Ser : Serine Ser(Bzl) : O-benzylserine Thr : Threonine Thr(Bzl) : O-benzylthreonine Pro : Proline Tpr : Thioproline Hyp : 4-hydroxyproline Hyp(Bzl) : 4-benzyloxyproline Azc : Azetidine-2-carboxylic acid Pip : Pipecolic acid (piperidine-2-carboxylic acid) Phe : Phenylalanine N-MePhe : N-methylphenylalanine Tyr : Tyrosine Trp : Tryptophan Trp (Me) : N in -methyltryptophan Trp (For) : N in -formyltryptophan Trp (Ac) : N in -acetyltryptophan Thg(2) : 2-(2-thienyl)glycine The protecting groups and reagents often used in the present specification are symbolized as follows: AcOEt : Ethyl acetate Boc : Tertiary butoxycarbonyl Bzl : Benzyl BrZ : 2-bromobenzyloxycarbonyl ClZ : 2-chlorobenzyloxycarbonyl Tos : p-toluenesulfonyl For : Formyl OBzl : Benzyl ester OPac : Phenacyl ester ONB : HONB ester TFA : Trifluoroacetic acid TEA : Triethylamine DIEA : Diisopropylethylamine IBCF : Isobutyl chloroformate DMF : N,N-dimethylformamide DCC : N,N'-dicyclohexylcarbodiimide DCU : N,N'-dicyclohexylurea HONB : N-hydroxy-5-norbornane-2,3-dicarboxyimide HOBt : N-hydroxybenzotriazole DCM : Dichloromethane THF : Tetrahydrofuran The cyclic hexapeptide represented by the formula (I) or a salt thereof of the present invention, can be produced using an ordinary means of peptide synthesis. Although this production can be achieved by liquid phase synthesis or solid phase synthesis, the former is preferred in some cases. Any optionally chosen known method can be used for this peptide synthesis. Such methods include those described by M. Bodansky and M.A. Ondetti in Peptide Synthesis, Interscience Publishers, New York (1966), by F.M. Finn and K. Hofmann in The Proteins, Vol. 2, edited by H. Nenrath and R.L. Hill, Academic Press Inc., New York (1976), by Nobuo Izumiya et al. in Peptide Gosei No Kiso To Jikken (in Japanese), Maruzen (1985), by H. Yajima, S. Sakakibara et al. in Seikagaku Jikken Koza 1, edited by the Japanese Biochemical Society, Tokyo Kagaku Dojin (1977), by T. Kimura et al. in Zoku Seikagaku Jikken Koza 2, edited by the Japanese Biochemical Society, Tokyo Kagaku Dojin (1987), and by J.M. Stewart and J.D. Young in Solid Phase Peptide Synthesis, Pierce Chemical Company, Illinois (1984), respectively known as the azide method, the chloride method, the acid anhydride method, the mixed acid anhydride method, the DCC method, the active ester method, the method using Woodward reagent K, the carbonylimidazole method, the oxidation reduction method, the DCC/HONB method, and the method using BOP reagent. For example, a cyclic hexapeptide of the formula (I) or a salt thereof can be produced by condensing a starting material having a reactive carboxyl group, which corresponds to one of two fragments of a cyclic hexapeptide represented by the formula (I) as defined above, divided at optionally chosen positions, and another starting material having a reactive amino group, which corresponds to the other fragment, and cyclizing and condensing the resulting linear hexapeptide, by a known condensing method. When the resulting condensation product has a protecting group, the protecting group is eliminated by a conventional method. In solid synthesis, in particular, the cyclic hexapeptide of the formula (I) or a salt thereof can be produced by binding an amino acid having a protected functional group not involved in the reaction and an insoluble carrier, such as Pam resin, via the carboxyl group of the amino acid, eliminating the amino-protecting group, then condensing the product with an amino acid having a protected functional group not involved in the reaction, repeating this cycle until the desired protected peptide is obtained, followed by protecting-group elimination by a conventional means, such as treatment with hydrogen fluoride, trifluoromethanesulfonic acid or trifluoroacetic acid, and simultaneous breakage of the bond with the insoluble carrier. The first and second starting materials mentioned above are normally amino acids and/or peptide fragments; when they are bound together, the desired cyclic hexapeptide of the formula (I) or a salt thereof is formed. The starting amino acids or peptide fragments are normally linear or branched. The "reactive carboxyl group" is defined as the carboxyl group as such, or as activated. The "reactive amino group" is defined as the amino group as such, or as activated. Usually, one of the above two functional groups involved in condensation has been activated. Said carboxyl group and amino group not involved in the condensing reaction were protected before initiation of the condensing reaction. With respect to the protection of functional groups not involved in the starting material reaction, the protecting groups used, elimination of the protecting groups, activation of the functional groups involved in the reaction etc., known functional groups or known means can be chosen as appropriate. Protecting groups for the amino group of the starting material include benzyloxycarbonyl, tertiary butyloxycarbonyl, tertiary amyloxycarbonyl, isobornyloxycarbonyl, 4-methoxybenzyloxycarbonyl, 2-chlorobenzyloxycarbonyl, adamantyloxycarbonyl, trifluoroacetyl, phthalyl, formyl, 2-nitrophenylsulfenyl, diphenylphosphonothioyl and 9-fluorenylmethyloxycarbonyl. Protecting groups for the carboxyl group include alkyl esters (e.g., ester groups of methyl, ethyl, propyl, butyl, tertiary butyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl and 2-adamantyl), benzyl ester, 4-nitrobenzyl ester, 4-methoxybenzyl ester, 4-chlorobenzyl ester, benzhydryl ester, phenacyl ester, benzyloxycarbonylhydrazide, tertiary butyloxycarbonylhydrazide and tritylhydrazide. Protecting groups for the indole of tryptophan include formyl, 2,4,6-trimethylbenzenesulfonyl, 2,4,6-trimethoxybenzenesulfonyl, 4-methoxy-2,3,6-trimethylbenzenesulfonyl, 2,2,2-trichloroethyloxycarbonyl and diphenylphosphonothioyl. However, the indole of tryptophan may not be protected. Starting materials having an activated carboxyl group include corresponding acid anhydrides, azides and active esters [esters with alcohols (e.g., pentachlorophenol, 2,4,5-trichlorophenol, 2,4-dinitrophenol, cyanomethyl alcohol, p-nitrophenol, N-hydroxy-5-norbornane-2,3-dicarboxyimide, N-hydroxysuccinimide, N-hydroxyphthalimide, N-hydroxybenzotriazole)]. Starting materials having an activated amino group include corresponding phosphoric acid amides. This condensing reaction can be carried out in the presence of a solvent. The solvent is chosen as appropriate from among solvents known to be useful in peptide condensation. Such solvents include anhydrous or hydrated dimethylformamide, dimethyl sulfoxide, pyridine, chloroform, dioxane, dichloromethane, tetrahydrofuran, acetonitrile, ethyl acetate, N-methylpyrrolidone, and appropriate mixtures thereof. Reaction temperature is chosen as appropriate over the range known to be useful for peptide bond formation reaction, normally about -20 to 30°C. Intramolecular cyclization can be carried out at any position in the peptide by a known method. For example, the terminal α-carboxyl-protecting group of the C-terminal amino acid of the protected peptide is first eliminated by a known method, followed by activation by a known method, after which the terminal α-amino acid residue of the N-terminal amino acid is eliminated by a known method and the peptide is intramolecularly cyclized at the same time. Alternatively, the terminal α-carboxyl-protecting group of the C-terminal amino acid of the protected peptide and the terminal α-amino-protecting group of the N-terminal amino acid are eliminated at the same time, after which the peptide is intramolecularly cyclized by a known condensing reaction. It is preferable to carry out the intramolecular cyclization under high dilution conditions in some cases. Useful methods of protecting group elimination include catalytic reduction in a hydrogen stream in the presence of a catalyst such as palladium black or palladium carbon, acid treatment with anhydrous hydrogen fluoride, methanesulfonic acid, trifluoromethanesulfonic acid, trifluoroacetic acid or a mixture thereof, and reduction with sodium in liquid ammonia. In protecting group elimination by the above acid treatment, normally carried out at temperatures between -20 and 40°C, it is effective to add a cation capturing agent, such as anisole, phenol, thioanisole, metacresol, p-cresol, dimethyl sulfide, 1,4-butanediol or 1,2-ethanedithiol. The 2,4-dinitrophenyl group used to protect the imidazole of histidine is eliminated by thiophenol treatment. The formyl group used to protect the indole of tryptophane is eliminated by alkaline treatment, such as with dilute sodium hydroxide or dilute ammonia, as well as by the above-described acid treatment in the presence of 1,2-ethanedithiol or 1,4-butanedithiol. Cyclic hexapeptide (I) thus produced is collected by ordinary means of peptide separation and purification, such as extraction, distribution, reprecipitation, recrystallization, column chromatography and high performance liquid chromatography, after completion of the reaction. Cyclic hexapeptide (I) of the present invention can be obtained as a metal salt (e.g., sodium salt, potassium salt, calcium salt, magnesium salt, zinc salt), a'salt with a base or basic compound (e.g., ammonium salt, arginine salt), an acid adduct salt, particularly a pharmacologically acceptable acid adduct salt, such as a salt with an inorganic acid (e.g., hydrochloric acid, sulfuric acid, phosphoric acid) or an organic acid (e.g., acetic acid, propionic acid, citric acid, tartaric acid, malic acid, oxalic acid, methanesulfonic acid), by a known method. The cyclic hexapeptide (I) or a salt thereof of the present invention thus obtained exhibits excellent antagonistic action against endothelin receptors (i.e., both endothelin A receptors and endothelin B receptors), particularly against endothelin B receptors. That is, the cyclic hexapeptide (I) or a salt thereof antagonizes both endothelin A receptors and endothelin B receptors, more potently antagonizing endothelin B receptors than do conventional endothelin receptor antagonists. Also, the cyclic hexapeptide (I) or a salt thereof may antagonize endothelin B receptors, while showing almost no antagonistic action against endothelin A receptors. In other words, the cyclic hexapeptide (I) or a salt thereof is more selective for endothelin B receptors, in comparison with conventional endothelin receptor antagonists. Specifically, the cyclic hexapeptide (I) or a salt thereof is characterized by 1) more potent antagonistic action against endothelin B receptors (e.g., lower IC 50 values), in comparison with the peptide produced in Examples of EP0528312A2, or 2) more potent antagonistic action against endothelin B receptors than against endothelin A receptors (e.g., lower ratios of IC50 values for endothelin B receptors/IC 50 values for endothelin A receptors), in comparison with the peptides disclosed in the Examples given in the specification for EP-A-0528312. For example, the cyclic hexapeptide (I) or a salt thereof is equivalent in IC 50 values for endothelin B receptors and higher in IC 50 values for endothelin A receptors, in comparison with conventional endothelin receptor antagonists. Therefore, the cyclic hexapeptide (I) or a salt thereof is capable of more efficiently suppressing vascular smooth muscle contractions mediated by endothelin B receptors, in comparison with conventional endothelin receptor antagonists. Because the cyclic hexapeptide (I) or a salt thereof antagonizes endothelin A receptors, although this antagonistic action is less potent than its antagonistic action against endothelin B receptors, it is capable of suppressing vascular smooth muscle contractions respectively mediated by endothelin A receptors and endothelin B receptors, even when used alone. However, when used in combination with the peptides disclosed in the Examples given in the specification for EP0528312A2, which excellently antagonizes endothelin A receptors, the cyclic hexapeptide (I) or a salt thereof is capable of more effectively suppressing vascular smooth muscle contractions respectively mediated by endothelin A receptors-and endothelin B receptors. The cyclic hexapeptide (I) or a salt thereof is therefore useful as an endothelin receptor antagonist (endothelin B receptor antagonist, in particular) in warm-blooded animals (e.g., rats, mice, rabbits, chickens, pigs, sheep, bovines, monkeys, humans). The cyclic hexapeptide (I) or a salt thereof is a safe peptide of low toxicity. Also, because the cyclic hexapeptide (I) or a salt thereof possesses excellent endothelin antagonistic activity as described above, it can be used as a therapeutic agent for diseases caused by endothelin, such as hypertension, pulmonary hypertension, angina pectoris, myocardiopathy, myocardial infarction, Raynaud's disease, Berger's disease, cerebral infarction, cerebral vascular spasms, asthma, acute renal failure, cyclosporine- or cisplatin-associated renal dysfunction, arteriosclerosis, diabetic nephropathy, diabetic neurosis, diabetic retinopathy, hemorrhagic shock, endotoxin shock, dysfunction of organs (e.g., liver) in organ surgery or transplantation, and hyperlipidemia. When the cyclic hexapeptide (I) or a salt thereof is non-orally administered as an endothelin antagonist, it is normally used in the form of a liquid (e.g., injection). Injectable preparations include intravenous, subcutaneous, intracutaneous, intramuscular and drip infusion injections. Such injections are prepared by a known method, i.e., by dissolving, suspending or emulsifying a compound possessing endothelin receptor antagonistic action in a sterile aqueous or oily solution. Aqueous solutions for injection include physiological saline, and isotonic solutions containing glucose and other auxiliary agents (e.g., D-sorbitol, D-mannitol, sodium chloride), and may be used in combination with appropriate dissolution aids, such as alcohols (e.g., ethanol), polyalcohols (e.g., propylene glycol, polyethylene glycol), and nonionic surfactants (e.g., polysorbate 80, HCO-50). Oily solutions include sesame oil and soybean oil, and may be used in combination with dissolution aids, such as benzyl benzoate and benzyl alcohol. Buffers (e.g., phosphate buffer, sodium acetate buffer), soothing agents (e.g., benzalkonium chloride, procaine hydrochloride), stabilizers (e.g., human serum albumin, polyethylene glycol), preservatives (e.g., benzyl alcohol, phenol) and other additives may also be added. The injection thus prepared is normally packed in an appropriate ampule. The dose per administration is normally 0.01 to 100 mg, preferably 0.01 to 50 mg, and more preferably 0.01 to 20 mg per kg body weight, depending on subject, target organ, symptoms, method of administration and other factors, in the case of an injection; it is advantageous to give this dose by intravenous injection. For oral administration, the cyclic hexapeptide (I) or a salt thereof is administered in the form of an oral preparation, such as powders, tablets, granules or capsules. In producing such an oral preparation, a pharmaceutically acceptable carrier may be added. Such carriers include excipients (e.g., lactose, starch), lubricants (e.g., magnesium stearate, talc), binders (hydroxypropyl cellulose, hydroxypropylmethyl cellulose, macrogol) and disintegrating agents (e.g., starch, carboxymethyl cellulose calcium). Antiseptics (e.g., benzyl alcohol, chlorobutanol, methyl p-oxybenzoate, propyl p-oxybenzoate), antioxidants, coloring agents, sweetening agents and other additives may also be used as necessary. The dose per administration is normally 5 mg to 1 g, preferably 10 to 100 mg per kg body weight. The cyclic hexapeptide (I) or a salt thereof can also be administered as a sustained-release preparation as disclosed in Japanese Patent Application No. 153393/1993. Specifically, the cyclic hexapeptide (I) or a salt thereof can be administered in a sustained-release preparation along with a biodegradable polymer (e.g., copolymer of fatty acid polyester or glycolic acid and lactose). [Examples] The present invention is hereinafter described in more detail by means of the following examples. Example 1 Production of cyclo[-D-Asp-Asp(B7)-Asp-D-γMeLeu-L-Leu-D-Trp-] (1) Production of Boc-D-γMeLeu-OH 4.80 g (33.06 mmole) of H-D-γMeleu-OH (produced by Watanabe Chemical Industries, Ltd.) was dissolved in a mixed solvent of 60 ml of dioxane and 30 ml of water. To this solution, 60 ml of a dioxane solution of 7.94 g (36.4 mmole) of (Boc) 2 O and 33.06 ml of a 1 M aqueous sodium hydroxide solution were simultaneously added drop by drop at the same rate under ice cooling conditions, followed by overnight stirring. After the reaction mixture was concentrated to 30 ml, 200 ml of water and 200 ml of ethyl acetate were added; the mixture was adjusted to pH about 2 with 6 N hydrochloric acid under ice cooling conditions to extract the desired product in the ethyl acetate layer. This extract was twice washed with 100 ml of dilute hydrochloric acid, after which it was washed with water to obtain a nearly neutral pH value and dried over Na 2 SO 4 , and the solvent was distilled off under reduced pressure. Petroleum ether was added to the residue; the resulting precipitate was collected by filtration to yield a white powder. Yield : 7.08 g (recovery 87.3%) m.p. : 98-99°C Rf 1 : 0.14 Rf 2 : 0.62 [α] D 25 : 15.6° (c = 1.16, DMF) Elemental analysis (for C 12 H 23 N 1 O 4 ): Calculated C, 58.75; H, 9.45; N, 5.71 Found C, 58.96; H, 9.68; N, 5.75 (2) Production of Boc-D-γMeLeu-Leu-OBzl 10.39 g (26.4 mmole) of pTos·H-Leu-OBzl was dissolved in 30 ml of DMF. To this solution, 4.82 ml (27.7 mmole) of diisopropylethylamine was added under ice cooling conditions, after which Boc-D-γMeLeu-ONH (prepared from 6.80 g (27.7 mmole) of Boc-D-γMeLeu-OH as synthesized in (1) above, 5.22 g (29.1 mmole) of HONB and 6.01 g (29.1 mmole) of DCC) was added, followed by overnight stirring. After the solvent was distilled off under reduced pressure, the residue was dissolved in 200 ml of ethyl acetate. This solution was washed with a 10% aqueous citric acid solution, saturated saline, a 4% aqueous NaHCO 3 solution and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was recrystallized from petroleum ether to yield a colorless crystal. Yield : 11.0 g (recovery 92.5%) m.p. : 93-94°C Rf 1 : 0.74 Rf 2 : 0.84 [α] D 25 : 8.9° (c = 1.00, DMF) Elemental analysis (for C 25 H 40 N 2 O 5 ): Calculated C, 66.93; H, 8.99; N, 6.24 Found C, 66.98; H, 9.00; N, 6.29 (3) Production of Boc-D-γMeLeu-Leu-OPac 10.8 g (24.0 mmole) of Boc-D-γMeLeu-Leu-OBzl as synthesized in (2) above was dissolved in 400 ml of methanol. To this solution, 1 g of palladium black was added, followed by stirring in a hydrogen stream at normal temperature and normal pressure for 5 hours. After the catalyst was filtered off, the solvent was concentrated under reduced pressure to about 100 ml. To this concentrate, 10 ml of an aqueous solution of 3.91 g (12.0 mmole) of Cs 2 CO 3 was added drop by drop over a period of 10 minutes. After additional stirring for 30 more minutes, the solvent was distilled off. To the residue, 50 ml of DMF was added; the solvent was distilled off under reduced pressure. This procedure was twice repeated. The residue was dissolved in 50 ml of DMF. To this solution, 50 ml of a DMF solution of 5.02 g (25.2 mmole) of phenacyl bromide was added drop by drop under ice cooling conditions over a period of 10 minutes, followed by overnight stirring at room temperature. After the solvent was distilled off under reduced pressure, the residue was dissolved in ethyl acetate. This solution was washed with a 10% aqueous citric acid solution, saturated saline, a 4% aqueous NaHCO 3 solution and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was crystallized from petroleum ether to yield a colorless crystal. Yield : 10.9 g (recovery 94.9%) m.p. : 57-58°C Rf 1 : 0.64 Rf 2 : 0.78 [α] D 25 : 3.7° (c = 1.14, DMF) Elemental analysis (for C 26 H 40 N 2 O 6 ): Calculated C, 65.52; H, 8.46; N, 5.88 Found C, 65.35; H, 8.65; N, 5.79 (4) Production of Boc-Asp(OBzl)-D-γMeLeu-Leu-OPac To 10.5 g (22.0 mmole) of Boc-D-γMeLeu-Leu-OPac as synthesized in (3) above, 40 ml of 10 N HCl/dioxane was added, followed by stirring for 15 minutes. After the solvent was distilled off under reduced pressure, ether was added; the resulting precipitate was collected by filtration, and dried. This dry product was dissolved in 30 ml of DMF. While this solution was cooled with ice, 4.02 ml (23.1 mmole) of diisopropylethylamine was added. To this mixture, Boc-Asp(OBzl)-ONB (prepared from 7.83 g (24.2 mmole) of Boc-Asp(OBzl)-OH, 4.77 g (26.6 mmole) of HONB and 5.49 g (26.6 mmole) of DCC) was added, followed by overnight stirring. After the resulting insoluble matter was filtered off, the filtrate was concentrated; the residue was dissolved in ethyl acetate, washed with a 10% aqueous citric acid solution, saturated saline, a 4% aqueous NaHCO 3 solution and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was crystallized from ether-petroleum ether to yield a colorless crystal. Yield : 13.5 g (recovery 90.0%) m.p.: 123-124°C Rf 1 : 0.57 Rf 2 : 0.76 [α] D 25 : -19.6° (c = 1.19, DMF) Elemental analysis (for C 37 H 51 N 3 O 9 ): Calculated C, 65.18; H, 7.54; N, 6.16 Found C, 65.10; H, 7.29; N, 6.10 (5) Production of Boc-Asp(B7)-Asp(OBzl)-D-γMeLeu-Leu-OPac 4.12 g (18 mmole) of 1-(2-methoxyphenyl)piperazine (produced by Aldrich) was dissolved in 30 ml of DMF. While this solution was cooled with ice, 10 ml of a DMF solution containing Boc-Asp(ONB)-OBzl (prepared from 4.85 g (15 mmole) of Boc-Asp-OBzl, 2.97 g (16.5 mmole) of HONB and 3.40 g (16.5 mmole) of DCC) was added drop by drop, followed by overnight stirring at room temperature. After the solvent was distilled off under reduced pressure, the residue was dissolved in 50 ml of ethyl acetate. This solution was washed with a 10% aqueous citric acid solution, saturated saline, a 4% aqueous NaHCO 3 solution and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was dissolved in 20 ml of a 1:1 mixture of hexane and ethyl acetate, and purified by silica gel column chromatography (5 cm dia. × 10 cm, hexane:ethyl acetate = 7:3). 1.81 g (3.63 mmole) of the resulting oily Boc-Asp(B7)-OBzl was dissolved in 50 ml of methanol. To this solution, 0.1 g of palladium black was added, followed by stirring in a hydrogen stream at normal temperature and normal pressure for 2 hours. After the catalyst was filtered off, the solvent was distilled off under reduced pressure. The residue was dissolved in 20 ml of acetonitrile and treated with 715 mg (3.99 mmole) of HONB and 824 mg (3.99 mmole) of DCC to yield Boc-Asp(B7)-ONB. To 1.48 g (3.63 mmole) of Boc-Asp(OBzl)-D-γMeLeu-Leu-OPac as synthesized in (4) above, 20 ml of 10 N HCl/dioxane was added under ice cooling conditions, followed by stirring for 15 minutes. After the solvent was distilled off under reduced pressure, ether was added; the resulting precipitate was collected by filtration and dried. This dry product was dissolved in 10 ml of DMF. While this solution was cooled with ice, 0.60 ml (3.47 mmole) of diisopropylethylamine was added. To this mixture, Boc-Asp(B7)-ONB as prepared above was added, followed by overnight stirring. After the resulting insoluble matter was filtered off, the filtrate was concentrated; the residue was dissolved in ethyl acetate. To this solution, 0.27 ml (2.08 mmole) of N,N-dimethyl-1,3-propanediamine was added, followed by stirring for 5 minutes. This mixture was washed with a 10% aqueous citric acid solution, saturated saline, a 4% aqueous NaHCO 3 solution and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was precipitated by addition of petroleum ether to yield a white powder. Yield : 2.65 g (recovery 82.7%) m.p. : 84-87°C Rf 1 : 0.40 Rf 2 : 0.71 [α] D 25 : -48.3° (c = 1.18, DMF) Elemental analysis (for C 52 H 70 N 6 O 12 ): Calculated C, 64.31; H, 7.27; N, 8.65 Found C, 64.14; H, 7.37; N, 8.71 (6) Production of Boc-D-Asp(OBzl)-Asp(B7)-Asp(OBzl)-D-γMeLeu-Leu-OPac To 2.50 g (2.58 mmole) of Boc-Asp(B7)-Asp(OBzl)-D-γMeLeu-Leu-OPac as synthesized in (5) above, 20 ml of 10 N HCl/dioxane was added under ice cooling conditions, followed by stirring for 15 minutes. After the solvent was distilled off under reduced pressure, ether was added; the resulting precipitate was collected by filtration and dried. This dry product was dissolved in 5 ml of DMF. While this solution was cooled with ice, 0.47 ml (5.42 mmole) of diisopropylethylamine was added. To this mixture, Boc-D-Asp(OBzl)-ONB (prepared from 918 mg (2.84 mmole) of Boc-D-Asp(OBzl)-OH, 559 mg (3.12 mmole) of HONB and 644 mg (3.12 mmole) of DCC) was added, followed by overnight stirring. After the resulting insoluble matter was filtered off, the filtrate was concentrated; the residue was dissolved in ethyl acetate. To this solution, 0.27 ml (2.08 mmole) of N,N-dimethyl-1,3-propanediamine was added, followed by stirring for 5 minutes. This mixture was washed with a 10% aqueous citric acid solution, saturated saline, a 4% aqueous NaHCO 3 solution and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was precipitated with petroleum ether to yield a white powder. Yield : 2.59 g (recovery 85.3%) m.p. : 81-83°C Rf 1 : 0.40 Rf 2 : 0.74 [α] D 25 : -36.9° (c = 1.25, DMF) Elemental analysis (for C 63 H 81 N 7 O 15 ): Calculated C, 64.32; H, 6.94; N, 8.33 Found C, 64.25; H, 7.10; N, 8.28 (7) Production of Boc-D-Trp-D-Asp(OBzl)-Asp(B7)-Asp(OBzl)-D-γMeLeu-Leu-OPac To 1.18 g (1.00 mmole) of Boc-D-Asp(OBzl)-Asp(B7)-Asp(OBzl)-D-γMeLeu-Leu-OPac as synthesized in (6) above, 20 ml of 10 N HCl/dioxane was added under ice cooling conditions, followed by stirring for 15 minutes. After the solvent was distilled off under reduced pressure, ether was added; the resulting precipitate was collected by filtration and dried. This dry product was dissolved in 4 ml of DMF. While this solution was cooled with ice, 0.36 ml (2.10 mmole) of diisopropylethylamine was added. To this mixture, Boc-D-Trp-ONB (prepared from 335 mg (1.10 mmole) of Boc-D-Trp-OH, 217 mg (1.21 mmole) of HONB and 250 mg (1.21 mmole) of DCC) was added, followed by overnight stirring. After the resulting insoluble matter was filtered off, the filtrate was concentrated; the residue was dissolved in ethyl acetate. To this solution, 0.08 ml (0.660 mmole) of N,N-dimethyl-1,3-propanediamine was added, followed by stirring for 5 minutes. This mixture was washed with a 10% aqueous citric acid solution, saturated saline, a 4% aqueous NaHCO 3 solution and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was precipitated by addition of ethyl acetate-petroleum ether to yield a white powder. Yield : 1.22 g (recovery 89.5%) m.p. : 94-97°C Rf 1 : 0.31 Rf 2 : 0.64 [α] D 25 : -22.2° (c = 1.08, DMF) Elemental analysis (for C 74 H 91 N 9 O 16 ): Calculated C, 65.23; H, 6.73; N, 9.25 Found C, 65.04; H, 6.97; N, 9.11 (8) Production of Boc-D-Trp-D-Asp(OBzl)-Asp(B7)-Asp(OBzl)-D-γMeLeu-Leu-OH 1.09 g (0.80 mmole) of Boc-D-Trp-D-Asp(OBzl)-Asp(B7)-Asp(OBzl)-D-γMeLeu-Leu-OPac as synthesized in (7) above was dissolved in 20 ml of a 90% aqueous solution of acetic acid. To this solution, 2.62 g (40 mmole) of zinc powder was added, followed by stirring for 1 hour. After the zinc powder was filtered off, the filtrate was concentrated; the residue was dissolved in dichloromethane, washed with dilute sulfuric acid and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was precipitated by addition of petroleum ether-ether to yield a white powder. Yield : 767 mg (recovery 77.0%) m.p. : 104-106°C Rf 1 : 0.18 Rf 2 : 0.62 [α] D 25 : -16.6° (c = 1.41, DMF) Elemental analysis (for C 66 H 85 N 9 O 15 ): Calculated C, 63.70; H, 6.88; N, 10.13 Found C, 63.55; H, 6.94; N, 9.99 (9) Production of cyclo[-D-Asp-Asp(B7)-Asp-D-γMeLeu-Leu-D-Trp-] 600 mg (0.48 mmole) of Boc-D-Trp-D-Asp(OBzl)-Asp(B7)-Asp(OBzl)-D-γMeLeu-Leu-OH as synthesized in (8) above was dissolved in 5 ml of DMF. While this solution was cooled with ice, 173 mg (0.96 mmole) of HONB and 199 mg (0.96 mmole) of DCC were added, followed by overnight stirring at 4°C. After the resulting insoluble matter was filtered off, the solvent was distilled off under reduced pressure. To the residue, 5 ml of 10 N HCl/dioxane was added under ice cooling conditions, followed by stirring for 15 minutes. After the reaction mixture was concentrated, ether was added to the residue; the resulting precipitate was collected by filtration and dried. This dry product was dissolved in 10 ml of DMF; the solution was added drop by drop to 90 ml of DMF containing 0.42 ml (2.41 mmole) of diisopropylethylamine, followed by stirring at room temperature for 3 hours. After the solvent was distilled off under reduced pressure, the residue was dissolved in ethyl acetate. This solution was washed with a 10% aqueous citric acid solution, saturated saline, a 4% aqueous NaHCO 3 solution and saturated saline, and dried over Na 2 SO 4 . After the solvent was distilled off, the residue was precipitated with ether-petroleum ether; the resulting precipitate was collected by filtration. 400 mg of the precipitate was dissolved in 20 ml of DMF. To this solution, 0.1 g of palladium black was added, followed by stirring in a hydrogen stream at normal temperature and normal pressure for 2 hours. After the catalyst was filtered off, the solvent was distilled off under reduced pressure. After addition of water, the residue was lyophilized, to yield 360 mg of a white powder. 68.4 mg of this powder was purified by reverse phase liquid chromatography (column YMC D-ODS-5 (2 cm × 25 cm), eluent 0.1% TFA-containing acetonitrile solution (37%-47% acetonitrile linear density gradient/30 minutes). Yield: 22.0 mg (preparatory HPLC recovery 32%) LSIMS (M + H) + = 946 (calculated value = 946) Example 2 The following peptide was produced by the method of Example 1. LSIMS (M + H) + = 916 (calculated value = 916) Example 3 The following peptide was produced by the method of Example 1. LSIMS (M + H) + = 988 (calculated value = 988) Example 4 Production of cyclo[-D-Asp-Asp(B7)-Asp-D-γMeLeu-Leu-D-Trp-]·2Na 4.4 g of cyclo[-D-Asp-Asp(B7)-Asp-D-γMeLeu-D-Trp-] as prepared by the method of Example 1 was dissolved in 50 ml of methanol, concentrated, dissolved in 50 ml of methanol again, and then cooled with ice. To this solution, 46.4 ml of a 0.1 N aqueous sodium hydroxide solution was added drop by drop. After addition of a 0.1 N aqueous sodium hydroxide solution to obtain pH 7 to 8, the mixture was concentrated; after addition of distilled water, the concentrate was lyophilized. Elemental analysis (for C 47 H 61 N 9 O 12 Na 2 ·CF 3 CO 2 Na·0.5CH 3 CO 2 Na· 3H 2 O): Calculated C, 49.18; H, 5.65; N, 10.32 Found C, 49.08; H, 5.50; N, 10.33 Formulation Example 1 3.6 g of lactic acid-glycolic acid copolymer (lactic acid/glycolic acid=75/25 mol %, GPC weight-average molecular weight 15,038, GPC number-average molecular weight 5,195, produced by Wako Pure Chemical Industry) was dissolved in 6.6 g (5 ml) of dichloromethane. To this solution was added a solution of peptide as obtained in Example 4 (250 mg) and L-arginine (100 mg) in 0.5 ml of distilled water, and the mixture was stirred for 30 seconds using a homogenizer (Polytron) to yield a w/o emulsion. The emulsion was injected to 800 ml of a 0.1 % (w/w) aqueous solution of polyvinyl alcohol (EG-40, produced by The Nippon Synthetic Chemical Industry, Co., Ltd.), previously adjusted to 18°C, followed by stirring in a turbine homomixer at 6,000 rpm to yield a w/o/w emulsion, which was then stirred at a room temperature for 3 hours to volatile the dichloromethane and solidify the oil phase, which was then collected via centrifugation at 2,000 rpm using a centrifuge (05PR-22, Hitachi, Ltd.). The precipitate was again dispersed in distilled water, centrifuged and washed to remove the free drug. After 100 mg of D-mannitol was added to the collected microcapsules, the microcapsules were re-dispersed in a small amount of distilled water, and the dispersion was lyophilized to yield powdery microcapsules. The microcapsules thus obtained were homogenized and extracted in 0.1 M ammonium acetate solution containing 30% (v/v) acetonitrile for 3 hours, and then assayed by HPLC (high performance liquid chromatography). As the result, the content of peptide as obtained in Example 4 was 5.2 mg per 100 mg of microcapsules. Comparative Formulation 1 Powdery microcapsules were obtained in the same manner as Formuration Example 1, except that the peptide of Example 4 was not used. Reference Example 1 The following peptide was produced in accordance with Example 96 of EP-A-0528312. Test Example 1 Determination of endothelin receptor binding activity (1) Determination of ET A receptor binding activity A membrane fraction prepared from swine ventricular muscle was diluted to 12 µg/ml with an assaying buffer and dispensed to assaying tubes at 100 µl per tube. To this membrane fraction suspension were added 2 µl of a solution of endothelin-1 labeled with 100 pmole of radioactive iodine ([ 125 I]ET-1, produced by Amersham Medical Ltd.) and 3 µl of a solution of the subject compound at various concentrations, followed by incubation at 25°C for 1 hour. The suspension was then diluted with an ice-cooled assaying buffer and filtered through a glass filter GF/F (produced by Whatman K.K.). The membrane fraction containing endothelin A (ET A ) receptors and receptor-bound [ 125 I]ET-1 were recovered on the filter. The radioactivity retained on the filter was measured to obtain the amount of [ 125 I]ET-1 bound to the ET A receptors. The amount of [ 125 I]ET-1 bound in the presence of ET-1 in great excess as the test compound was taken as the amount of nonspecific binding. The amount of specific binding (SPB) of [ 125 I]ET-1 was obtained by subtracting the amount of nonspecific binding from the total amount of binding determined. The percent SPB value at test compound concentration C (molar concentration by volume) was defined as the percent ratio of the amount of specific binding relative to the amount of specific binding obtained at a test compound concentration of 0. The thus-obtained value was substituted for the following "pseudo-Hill's equation" to calculate the test compound concentration (IC 50 value) required to have a value of 50% for the amount of specific binding of [ 125 I]ET-1. log[%SPB/(100 - %SPB)] = n[logC - log(IC 50 )] n: Pseudo-Hill's coefficient (2) Determination of ET B receptor binding activity A membrane fraction prepared from a bovine brain was diluted to 150 µg/ml with an assaying buffer and dispensed to assaying tubes at 100 µl per tube. To this membrane fraction suspension were added 2 µl of a solution of endothelin-1 labeled with 100 pM of radioactive iodine ([ 125 I]ET-1, produced by Amersham Medical Ltd.) and 3 µl of a solution of the subject compound at various concentrations, followed by incubation at 25°C for 1 hour. The suspension was then diluted with an ice-cooled assaying buffer and filtered through a glass filter GF/F (produced by Whatman K.K.). The membrane fraction containing endothelin B (ET B ) receptors and receptor-bound [ 125 I]ET-1 were recovered on the filter. The radioactivity retained on the filter was measured to obtain the amount of [ 125 I]ET-1 bound to the ET B receptors. The amount of [ 125 I]ET-1 bound in the presence of ET-1 in great excess as the test compound was taken as the amount of nonspecific binding. The amount of specific binding (SPB) of [ 125 I]ET-1 was obtained by subtracting the amount of nonspecific binding from the total amount of binding determined. The percent SPB value at test compound concentration C (molar concentration by volume) was defined as the percent ratio of the amount of specific binding relative to the amount of specific binding obtained at a test compound concentration of 0. The thus-obtained value was substituted for the following "pseudo-Hill's equation" to calculate the test compound concentration (IC 50 value) required to have a value of 50% for the amount of specific binding of [ 125 I]ET-1. log[%SPB/(100 - %SPB)] = n[logC - log(IC 50 )] n: Pseudo-Hill's coefficient (3) Using the peptides of Examples 1 through 3 and Reference Example 1, IC 50 values for ET A receptors and ET B receptors were calculated. The results are given below. Peptide IC 50 (M) for ET A IC 50 (M) for ET B Example 1 1.8 × 10 -10 1.1 × 10 -8 Example 2 1.8 × 10 -10 2.8 × 10 -8 Example 3 4.9 × 10 -10 1.4 × 10 -8 Reference Example 1 0.8 × 10 -10 12.0 × 10 -8 These results demonstrate that the peptides of the present invention excellently antagonize ET A receptors and ET B receptors, and that they are more potent in ET B receptor antagonistic action than in ET A receptor antagonistic action, in comparison with the peptide disclosed in EP-A-0528312. Test Example 2 Determination of porcine coronary vein relaxation activity Ring preparations of porcine coronary vein (3 mm in a diameter) were placed in an organ bath filled with Krebs solution gassed with 95% O 2 /5%CO 2 at 37°C and allowed to stand for 1.5 h at a resting load of 0.5 g tension for vein. The preparations were constricted with 60 mM KCl for 15 min and then washed and allowed to stand for 1.5 h at the resting load. The vein preparations constricted with sarafotoxin S6c (S6c) at 1 nM for 45 min were used to determine the effect of the peptide of Example 4 at 0.01µM. Result is shown in the following Table as the relaxation rate relative to the constriction of S6c (100%). Peptide relaxation rate (%) Example 4 66 The result demonstrates that the peptide of the present invention excellently possesses a vascular smooth muscle relaxation activity. Test Example 3 A preparation as obtained in Formuration Example 1 or Comparative Formulation 1 as a control group was subcutaneously administered to the back of 8-week-old male Wistar fatty rats every four weeks. The male Wistar fatty rat, a line of rat which genetically develops obesity and hyperglycemia, is characterized by increased leakage of protein and albumin in urine with the development of hyperglycemia. A dosage of the preparation as obtained in Formulation Example 1 was 3 mg per one rat (58 mg per one rat as the microcapsules). A dosage of Comparative Preparation 1 was 200 mg per one rat. The results of urinary albumin assays in a control group and an administration group of Formuration Example 1 are given in Table 1. Urinary Albumin (mg/day, mean) Weeks after administration 0 3 6 Control group 3 10 27 Formulation Example 1 3 6 9 As seen in Table 1, during the period of about 6 weeks after Formulation Example 1 administration, smaller amounts of albumin were excreted in the urine, in comparison with control velues. These results demonstrate that urinary albumin excretion, a symptom of diabetic nephropathy, were suppressed during the perod when the endothelin antagonist was sustained in the live body as shown in Test Example 3, suggesting the utility of the compounds of the present invention for treating diabetic nephropathy.

Claims: A cyclic hexapeptide represented by the formula (I): cylco[-D-Asp-Asp(R 1 )-L-Asp-NH-CHR 2 -CO-L-Leu-D-Trp(N in -R 3 )-] wherein R 1 is a group represented by the formula: wherein X 1 is a hydrogen atom, a C 1-6 alkyl group, a C 1-6 alkoxy group, a halogen atom or a nitro group; R 2 is a neopentyl group; and R 3 is a hydrogen atom or -COR 4 (R 4 represents a hydrogen atom, a C 1-6 alkyl group, a C 6-15 aryl group or a C 6-15 aryl-C 1-3 alkyl group); or a salt thereof. The peptide of claim 1, wherein X 1 is a hydrogen atom. The peptide of claim 1, wherein X 1 is a C 1-6 alkoxy group. The peptide of claim 1, wherein X 1 is a methoxy group. The peptide of claim 1, wherein R 3 is a hydrogen atom. The peptide of claim 1, wherein R 3 is -COR 4 (R 4 represents a C 1-6 alkyl group). The peptide of claim 1, wherein R 3 is an acetyl group. The peptide of claim 1, wherein X 1 is a hydrogen atom or a C 1-6 alkoxy group, and R 3 is a hydrogen atom or -COR 4 (R 4 represents a hydrogen atom or a C 1-6 alkyl group). The peptide of claim 1, wherein the cyclic hexapeptide of the formula (I) is cyclo[-D-Asp-L-Asp(B1)-L-Asp-D-γMeLeu-L-Leu-D-Trp-] with Asp(B1) being aspartic acid β-4-phenylpiperazine amide and γMeLeu being gamma methylleucine. The peptide of claim 1, wherein the cyclic hexapeptide of the formula (I) is cyclo[-D-Asp-L-Asp(B7)-L-Asp-D-γMeLeu-L-Leu-D-Trp-] with Asp(B7) being aspartic acid β-4-(2-methoxyphenyl)-piperazine amide and γMeLeu being gamma methylleucine. The peptide of claim 1, wherein the cyclic hexapeptide of the formula (I) is cyclo[-D-Asp-L-Asp(B7)-L-Asp-D-γMeLeu-L-Leu-D-Trp(Ac)-] with Asp (B7) being aspartic acid β-4-(2-methoxyphenyl)piperazine amide, γMeLeu being gamma methylleucine and Trp(Ac) being N in -acetyltryptophan. A method of producing the cyclic hexapeptide or a salt thereof of claim 1, which method comprises condensing two kinds of fragments of a cyclic hexapeptide represented by the formula: cylco[-D-Asp-Asp(R 1 )-L-Asp-NH-CHR 2 -CO-L-Leu-D-Trp(N in -R 3 )-] wherein R 1 , R 2 and R 3 have the same meaning as in claim 1, divided at an optionally chosen position to form a linear hexapeptide, and then cyclizing the resulting linear hexapeptide to form the cyclic hexapeptide. Use of the cylcic hexapeptide of the formula (I) as defined in claim 1 or a pharmacologically acceptable salt thereof for the manufacture of a medicament for treating a disease caused by endothelin. The use of claim 13, wherein the disease is hypertension, pulmonary hypertension, myocardial infarction, cerebral infarction, cerebral vascular spasms, asthma, acute renal failure, arteriosclerosis, diabetic nephrophathy or dysfunction of organs in organ surgery or transplantation.

Inventors: KAWADA AKIRA, KIKUCHI TAKASHI, WAKIMASU MITSUHIRO, KAWADA, AKIRA, KIKUCHI, TAKASHI, WAKIMASU, MITSUHIRO
Applicants: TAKEDA CHEMICAL INDUSTRIES LTD, TAKEDA CHEMICAL INDUSTRIES, LTD.
Application year: 1994
Main classification: C07K14
IPC codes: C07K14, A61K38